Speed of Reaction

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Transcript of Speed of Reaction

Speed of ReactionRate of ReactionThe speed of different chemical reactions varies hugely. Some reactions are very fast and others are very slow.

The speed of a reaction is called the rate of the reactionEffect of temperature The higher the temperature, the faster the rate of a reaction. Effect of ConcentrationThe higher the concentration of a dissolved reactant, the faster the rate of a reaction.

When reactants are gases, the speed of the reaction increases as the pressure is increased.The greater the surface area, the faster the rate of a chemical reaction.Objectives to meet:defining the speed of reaction in terms of rate of reactionstating factors affecting speed of reactiondescribing the affect of temperature, pressure, concentration and particle size on the speed of reaction.explaining the collision theorydefine the term catalyst and enzyme and explain how they affect speed of reactiondescribe methods of measuring the speed of reactionsketch graphs of changes in mass or volume when gases are given off during reactionsinterpret data obtained from experiments concerned with speed of reaction

It means how fast something changes, and is obtained by dividing some quantity by a unit of time.

Velocity = distance / timeRateReactions, particles and collisions - The Collision theory:In order to react molecules and atoms must touch each other.They must hit each other hard enough to react.Reactions take place when particles collide with a certain amount of energy.

The rate of a reaction depends on two things:

the frequency of collisions between particles

&

the energy with which particles collide.

Changing the rate of reactionsAnything that increases the number of successful collisions between reactant particles will speed up a reaction.

Reactions do not proceed at a steady rate. They start off at a certain speed, then get slower and slower until they stop.

As the reaction progresses, the concentration of reactants decreases. This reduces the frequency of collisions between particles and so the reaction slows down.

What factors affect the rate of reactions?

increased temperatureincreased concentration of dissolved reactants increased pressure of gaseous reactantsincreased surface area of solid reactantsuse of a catalystIn many reactions, a rise in temperature of 10 °C causes the rate of reaction to approximately double.

Here's Why!At a higher temperature, particles have more energy. This means they move faster and are more likely to collide with other particles.When the particles collide, they do so with more energy, and so the number of successful collisions increases.

Increasing the temperature has the biggest effect on the rate of a reaction compared to the other factors.Here's Why!At a higher concentration, there are more particles in the same amount of space. This means that the particles are more likely to collide and therefore more likely to reactEffect of PressureHere's Why!As the pressure increases, the space in which the gas particles are moving becomes smaller.

The gas particles become closer together, the frequency of collisions. This means that the particles are more likely to react.

Effect of Particle SizeHere's Why!Any reaction involving a solid can only take place at the surface of the solid.

If the solid is split into several pieces, the surface area increases. This means that there is an increased area for the reactant particles to collide with.

The smaller the pieces, the larger the surface area. This means more collisions and a greater chance ofreaction.

Effect of CatalystsCatalysts are substances that change the rate of a reaction by increasing it.Catalyst achieve this without itself being chemically changed.Catalysts never produce more product – they just produce the same amount more quickly.

Recalling collision theory!Reactions can only happen when the reactant particles collide, but most collisions are NOT successful in forming product molecules despite the high rate of collisions.

The reason is that only a small fraction of particles have enough kinetic energy to break bonds and bring about chemical change.

The minimum kinetic energy required for reaction is known as the activation energy.In order for a reaction to proceed, the activation energy of the reaction must be overcome.How catalysts work!Catalysts lower the need of energy to break bonds so activation energy is lower. Consequently, bond breaking occurs easily and more often when particles collideOther characteristics of catalystscan be reused and only small amount of catalyst is needed to affect a reaction.

Measuring the speed of reactionVisible changes are used to monitor the speed of reaction.Measuring time for reaction to completeMeasuring the volume of gas evolvedMeasuring change in mass of reaction mixturetemperature changescolour changesprecipitate formationSpeed of reaction is inversely proportional to time taken; the shorter the time needed for reaction to complete, the faster the speed of reaction is.Measuring change in mass of reactants

CaCO3 + 2HCl → CaCl2 + H2O + CO2Speed of reaction can be monitored by measuring mass changes every min(or any specified interval)Marble is reacted with acid in a flask with cotton wool stuck at top to prevent splashing during reaction but it allows gas to be free. Measuring the volume of gas evolved a. Speed of reaction = volume of gas evolved/ time taken

b. Speed of reaction = mass of product produced or amount of a reactant remaining/